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From: Marko Rauhamaa <marko@pacujo.net>
Newsgroups: comp.lang.python
Subject: Re: Late-binding of function defaults (was Re: What is a function parameter =[] for?)
Date: Thu, 26 Nov 2015 10:27:13 +0200
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Chris Angelico <rosuav@gmail.com>:

> On Thu, Nov 26, 2015 at 5:52 PM, Marko Rauhamaa <marko@pacujo.net> wrote:
>> Nothing prevents using mutable objects as keys in Python.
>
> Sure, you _can_. But if the key's hash changes between dict insertion
> and retrieval, all manner of invariants will break, and likewise if
> two equal objects have different hashes. From which you can deduce
> logically that any object used as a key must remain (not) equal to
> everything that it was (not) equal to from that time until it is
> looked up... which basically means its value mustn't change. It must
> be immutable.

What I'm saying is that Python does not prevent mutable keys but tries
to do that with lists and tuples.

I think Python should stop trying.

I have wanted to use lists as keys, and there should be no reason to
allow mutable tuples. It should be enough to say that the behavior of a
dictionary is undefined if a key should mutate on the fly.


Marko